Increasing demands in fuel efficiency have made hybrid systems more attractive in the automotive industry. In addition to a conventional combustion engine, an electric machine, which serves as both motor and generator (commonly called electric motor) is an important part of the hybrid system. To reduce manufacturing cost, many of the electric motors used in the hybrid systems are induction motors. Alternating current (AC) induction motors are commonly used in hybrid vehicles because they offer simple, rugged construction, easy maintenance, and cost-effectiveness. The AC induction motor has two basic assemblies: a stator and a rotor. The name “induction motor” comes from the AC “induced” into the rotor via the rotating magnetic flux produced in the stator. An aluminum squirrel cage carries the electrical current, and high electrical conductivity is needed. The lamination steel in the rotor carries magnetic flux. The rotating magnetic field induces electrical current in the squirrel cage. The induced magnetic field in the rotor interacts with the offset magnetic field in the stator, and leads to rotation and the generation of torque. In operation, the rotor speed always lags the magnetic field's speed, allowing the rotor bars to cut the magnetic lines of force and to produce useful torque. This speed difference is called slip speed. Slip increases with load and is necessary for torque production.
The stator structure is typically composed of steel laminations shaped to form poles. Copper wires are formed and inserted as part of the stator assembly. They are connected to a voltage source to produce a rotating magnetic field.
The rotor is typically made of laminations over a steel shaft. The iron core (laminate steel stack) serves to carry the magnetic field across the motor. The structure and materials for the iron core are specifically designed to minimize magnetic losses. The thin laminations (steel sheets), separated by varnish insulation, reduce stray circulating currents that would result in eddy current loss. The material for the laminations typically is a low carbon, high silicon steel specially tailored to produce certain magnetic properties, such as inhibiting eddy currents and narrowing the hysteresis loop of the material (small energy dissipation per cycle, or low core loss) and high permeability (electromagnetism). The low carbon content makes it a magnetically soft material with low hysteresis loss. To reduce the air gap and core loss between the thin laminated steel sheets, it is desired to keep the laminate steel stack as tight as possible. In practice, the laminate steel stack is usually held together using point welding or an inter-lock mechanism.
Radial slots around the laminations' periphery house rotor bars, which are typically made of aluminum or copper. The rotor bars are often skewed slightly along the length of the rotor to reduce noise and to smooth out torque fluctuations that might result in some speed variations due to interactions with the pole pieces of the stator. The arrangement of the rotor bars resembles a squirrel cage.
Because of its high density and melting point, copper has limitations and/or unique problems in rotor applications, particularly for hybrid systems. In hybrid applications, a high speed (e.g., more than 10,000 rpm) electric motor is usually needed due to space limitations in automotive vehicles. High density copper can produce very high centrifugal force and inertia at high rpm, and may cause performance and durability issues. In addition, rotors are preferably manufactured by high pressure die casting (HPDC). The high melting point of copper (1083° C.) makes the casting process extremely difficult and significantly reduces die life and increases the manufacturing cost of copper rotors.
For the induction motor, an aluminum based squirrel cage is very commonly used because aluminum is much lighter and less expensive than copper. FIG. 1 illustrates a squirrel cage rotor. Although cast aluminum rotors overcome the shortfalls of high rotating inertia and low die life associated with copper material, the mechanical properties impose a great challenge for their successful application in electric motors. The electrical conductivity of aluminum is 37.8×106 S.M−1 (at 20° C.), compared with 59.6×106 S.M−1 for copper. Pure Aluminum (99.7% purity) has high electrical conductivity (61% of that of pure copper), but low mechanical properties. A6101-T61 (0.6 Mg-0.5 Si) has relatively high electrical conductivity (57%) and improved strength. Both materials are commonly used to make squirrel cages for induction motors. The material's composition, porosity, stress/strain curve, fatigue and creep resistance, and electrical conductivity are very important to the motor's performance and durability. Porosity, commonly seen in casting aluminum, can affect electrical conductivity. The aluminum alloys used for rotor applications are usually wrought alloys which are difficult to cast because their low fluidity, high shrinkage rate (density change from liquid to solid), high melting temperature, and large freezing range (temperature difference between liquidus and solidus), etc. These characteristics of the aluminum wrought alloys increase the porosity and the tendency of hot tearing, particularly for the locations between the conductive bars and the end rings, which lead to fracture of the end rings. Additionally, many cast aluminum rotors are made by high pressure die casting. The entrained air and abundant aluminum oxides produced during the high pressure die casting process, which is due to very high flow velocity (about 60 m/s) in mold filling, can also significantly reduce the thermal and electric conductivity of the rotor, particularly in rotor bars.
Therefore, there is a need for an improved rotor for induction motors and for methods of making them.